255 research outputs found
String stability and a delay-based spacing policy for vehicle platoons subject to disturbances
A novel delay-based spacing policy for the control of vehicle platoons is
introduced together with a notion of disturbance string stability. The
delay-based spacing policy specifies the desired inter-vehicular distance
between vehicles and guarantees that all vehicles track the same spatially
varying reference velocity profile, as is for example required for heavy-duty
vehicles driving over hilly terrain. Disturbance string stability is a notion
of string stability of vehicle platoons subject to external disturbances on all
vehicles that guarantees that perturbations do not grow unbounded as they
propagate through the platoon. Specifically, a control design approach in the
spatial domain is presented that achieves tracking of the desired spacing
policy and guarantees disturbance string stability with respect to a spatially
varying reference velocity. The results are illustrated by means of
simulations.Comment: 15 pages, 10 figure
Stability and String Stability Analysis of Formation Control Architectures for Platooning.
This thesis presents theoretical results for stability and string stability
of formation control architectures for platooning. We consider
three important interconnection topologies for vehicles travelling in a
straight line as a string: leader following, cyclic and bidirectional.
For the leader following topology we discuss modifications that allow
reduced coordination requirements. In the first case we consider
the use of the leader velocity as the state to be broadcast to the followers,
rather than the standard use of the leader position. This selection
yields a formation control architecture that achieves string stability
even under time delays in the state broadcast, while reducing typical
coordination requirements of leader following architectures. For the
second modification we change the way in which the leader position
is sent across the string to every follower. This technique keeps some
of the good transient properties of the standard leader following architecture
but eliminates most of the coordination requirements for
the followers. However, we show that this technique does not provide
string stability when time delays are present in the communication.
The second topology that we discuss is a cyclic one, where the first
member of the platoon is forced to track the last one. We discuss two
strategies: one where the inter-vehicle spacings may follow a constanttime
headway spacing policy and one where an independent leader
broadcasts its position to every member of a cyclic platoon. For both
strategies we obtain closed form expressions for the transfer functions
from disturbances to inter-vehicle spacings. These expressions allow
us to show that if the design parameters are not properly chosen, the
vehicle platoon may become unstable when the string size is greater
than a critical number. On the contrary, if the design parameters are
well chosen, both architectures can be made stable and string stable
for any size of the platoon.
The final topology that we consider is bidirectional, where every
member of the platoon, with the exception of the first and last, use
measurements of the two nearest neighbours to control their position
within the string. Although the derivations are more complex than
in the two previous unidirectional cases, we obtain closed form epressions for the dynamics of the platoon. These expressions are in
the form of simple transfer functions from disturbances to vehicles.
They allow us to obtain stability results for any size of the platoon
and understand the behaviour of the least stable pole location as the
string size increases.
All of the results obtained are illustrated by numerical examples
and ad-hoc simulations
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